It is customary to employ belt pagers and paging stations at remote sites or zones to communicate with personnel in the field for the purpose of transmitting information between a central office or terminal and each employee. Typically, each paging station includes an FM receiver and an inhibit circuit including an inhibit relay which is energized anytime a signal level is received above a selected detection threshold which will cause the relay to be energized and generate an inhibit signal. If, for instance, a paging computer terminal at the central office wishes to follow a command to deliver a message to an employee via the paging system, it will look first to determine if an inhibit is present in the employee's zone. If the inhibit is present, it follows its programming and skips the first zone, goes to the second zone and looks for an inhibit. If the inhibit signal had not been present in the first zone, then the next step would have been for the computer to send a command to turn on the remote radio transmitter and, after a slight delay, the actual data message is sent from the paging computer terminal.
The data message is supplied to the paging radio transmitter input circuitry, modulates the carrier frequency, then is applied to the antenna lead to the station antenna and transmitted to the employee's belt pager. The radio frequency of the radio paging transmitter and the employee's pager are the same. Thus, the belt pager will emit an alert tone whereupon the employee presses a button on the pager to read the message.
A problem associated with existing inhibit circuits is in discriminating between a valid radio signal received within a specific band width assigned to a receiver and one in an adjacent channel as well as noise or other interference and which may tend to overlap the assigned channel thereby causing the inhibit relay to be accidentally closed.
Representative patents in this area are U.S. Pat. Nos. 2,932,729 to S. Yamato et al; 3,456,252 to A. J. Pantos; 4,412,347 to J. B. Lipcon; 4,519,074 to P. C. Basile and 4,718,059 to B. Sqzga et al. Although these patents generally may be directed to inhibit circuitry to prevent reception of more than one signal at a time, none is designed to be used in FM radio receivers and specifically in such a way as to be capable of discriminating between noise and valid signals within a specific assigned frequency band or band width. In the past, crystal filters have been utilized between the antenna and FM receiver to differentiate between signals and assure that the signal is within a particular frequency band. Nevertheless, crystal filters are quite expensive and do not always afford the desired performance, particularly in the filter shape factor and response time when utilized in association with paging transmitters of the type described. Again, it is of particular importance that the inhibit circuit be so controlled as to be capable of discriminating between valid signals received within the band width assigned to its receiver and those of adjacent channels.